Optical processing applications in lightwave communications, optical computing and photonic switching are creating a need for high speed, high performance optical and photonic devices such as modulators and the like. While modulation-doped quantum well structures have been developed for such applications, the structures have relatively long charge depletion time which limits the fundamental or intrinsic speed of operation of the device. Quantum wells are understood in practice as being a narrow energy bandgap material layer sandwiched between wider energy bandgap material layers wherein the thickness of the narrow energy bandgap material layer is much less than twice the exciton Bohr radius (&lt;&lt;2a.sub.0) for the narrow energy bandgap material. In GaAs, for example, a narrow bandgap material layer between two wider bandgap material layers is considered a quantum well when its thickness is less than 300 .ANG..
Modulation-doped quantum well heterostructures have been presented in which the heterostructures are cascadable in a semiconductor device to achieve high speed operation while obtaining large optical effects such as index of refraction or absorption coefficient changes for modulating lightwave signals without significant increases in the operating potentials over prior quantum well structures. Each modulation-doped quantum well heterostructure exhibits substantially equal boundary conditions with respect to each end of the heterostructure in an unbiased condition for efficient cascading or stacking. Each quantum well is said to be in the active region of the heterostructure and has associated with it both an isolation barrier layer to minimize leakage current and, through a transfer barrier, a separate charge reservoir. The latter aspect contributes to the speed of the cascadable quantum well heterostructure. See, for example, an article describing this quantum well heterostructure by Wegener et al. in Phys. Rev. B., Vol. 41, pp. 3097-3104 (1990).
In contrast to former quantum well devices, the cascaded modulation-doped quantum well heterostructures are synchronously or substantially simultaneously depleted in the presence of a bias potential which tends to reduce the overall switching potentials. When incorporated within a waveguide structure, cascaded modulation-doped quantum well heterostructures can be used as a waveguide element or as an intra-cavity element such as a modulator for directly modulated light source. Due to relatively large electrically induced changes in optical characteristics, it is possible to fabricate short waveguide structures to produce a relatively large change in optical characteristics. While many attributes of the modulation-doped quantum well heterostructure device are appealing, it should be noted that any device incorporating quantum wells is polarization sensitive or, alternatively, polarization dependent as a result of valence band splitting associated with quantum wells. This presents possible problems for lightwave communication system applications. It should be noted that the valence band splitting occurs when the quantum layer thickness is significantly less than twice the exciton Bohr radius. Moreover, in order to deplete the quantum well of electrons, it is necessary to have the electrons surmount a finite potential barrier (a transfer barrier) which limits the ultimate speed of the device. The latter aspect is more noticeable when depleting carriers from each quantum well rather than supplying carriers to each quantum well.